Coil for induction furnaces



Aug. 29, 1933. w. ESMARCH COIL FOR INDUCTION FURNACES Filed March 6,1931 In 1167; i021- Wz'llzelm Esmarck 5y 156s fliioz'ize Patented Au 29,1933 COIL FOR INDUCTION FURNACES Wilhelm Esmarch, Berlin-Halensee,Germany,

assignor to Siemens & Halske, Aktiengesellschaft, Siemensstadt, nearBerlin, Germany, a' corporation of Germany Application March 6, 1931,Serial No. 520,503, and in Germany March 20, 1930 2 Claims. (01. 219-47)The invention relates to a novel construction of coil for inductionfurnaces, designed for high efficiency and low operating cost bycombining in said coil the advantages of the water .cooled 5 tubularconductor and of the thin metal band conductor, the latter serving 'tosubstantially eliminate theskin effect and the former serving as themeans for cooling.

The invention is illustrated in the accompanying drawing, in which:-

Fig. 1 is a developed view illustrating the elements of a coil inaccordance with the invention.

Fig. 2 is a sectional plan view of a typical furnace with a coilconstructed in accordance with the invention, associated therewith.

It has been customary to construct the coils of high frequency inductionfurnaces of a series of convolutions of water cooled copper tubing. Suchconstruction possesses the advantage of effecting the intensive coolingof the coil and the consequent employment of a heavy current. On theother hand, in windings of this kind, the influence of the skineflect'is highly detrimental, in that the current distributionthroughout the cross section of the conductor is unequal and most of thecurrent is crowded to the inside of the coil of the conductor and themajor part of the conductor is practically free from current. Because ofthese conditions, the losses in furnaces of this character are high andthe efficiency of the furnace is greatly impaired. In order to avoid theobjections inherent in the water cooled coils, in furnaces .of thistype, and to insure a more efficient distribution of the currentthroughout the cross section of the conductors of which the coil iscomposed, it has been proposed to subdivide the conductor forming thecoil into a series of relatively thin bands of metal, preferably copper,insulated from each other, the thickness of which bands is so regulatedthat the so-called skin effect is substantially eliminated. Theindividual copper bands are so disposed and interlaced that they areequivalent or alike in respect of the resistance, impedance andinductive action, so.

5 lar coils hereinbefore referred to.

that all of them are traversed by equal currents.

According to the present invention, the advantages of water cooling thecoils or the individual convolutions thereof may be effected by makingthe conductors of the coil in part of water cooled tubing and in part ofthe relatively thin bands of copper or the like, the tubular conductorswith thecirculating cooling water being disposed to the outside of thecoil, sothat the heat developed in the bands will be transferred to thewater in the tubular sections. All of the conducting elements, includingthe bands and the tubular sections, are electrically insulated from eachother either by suitable lacquer coatings, or, if necessary, by means ofinterposed strips or bands of insulation. The tests have shown that thinlayers of insulation are no obstacle to an eficient exchange of heatbetween the bands and the water cooled tubing and that this coolingaction may be eifected with as great a degree of intensiveness andefficiency as in the-case of the coils formed entirely of the watercooled copper tubing. On the other hand, the coil constructed inaccordance with the invention has the additional advantage of involvinga loss of current. Further, the manufacture of such a coil isconsiderably cheaper than that of the subdivided coils made of thinconductor strips, as heretofore proposed, because the coil of thepresent invention avoids the expense of the air cooling systemassociated with the strip coils and because, in most instances, asmaller number of conductors are required in the new form of coil toproduce the same effect.

The drawing illustrates an exemplary arrangement of furnace and coilgenerally similar to that shown in United States Patent No. 1,721,073,dated July 16, 1929, except that each convolution of the coil includes atubular water cooled section and two strip sections. According to thepatent, the high frequency induction furnaces employ for the primarywinding flat wound bands of conducting material insulated from eachother, the bands being relatively so thin that the skin effect issubstantially eliminated and the individual convolutions of the coilbeing so disposed as to be identical in respect of resistance, impedanceand inductive effect on the furnace charge. As stated, the presentinvention involves the application of the water cooling effect bytubular conductors and the elimination of the losses due to skin effect,by employing the thin band-like conductors.

As shownin the figures of the drawing, three separate convolutions ofthe coil are connected in parallel with the supply mains, each convoluition of a tubular conductor having connections at each end forcirculating water therethrough and two sections or, lengths of the thinstrip conductor. The relation of the conductors is indicated in thedevelopment in Fig. 1 and also in the plan view of the coil as appliedto the furnace in Fig. 2. Each section of the coil constitutes acomplete convolution, the portions 411, a: and a: or the respectivecoils being located adjacent theiouter wall of the furnace and nextthereto on the outside are the respective coil sections b1, b2 and b3and outside these are the tubular conductors 01, c: and ca, which arewater cooled by suitable connections with a water supply and exhaustsystem, whereby the water is supplied. The individual tubularconductors, 01, c: and c; are insulated from each other and aresubstantially equal in respect of resistance, impedance and inductiveaction with the individual coil parts '01, a: and b1, ha. The elementsof each convolution are connected up in the following order: Stripconductor sections 111 and b2 and tubular conductor c3; strip conductorb1, tubular conductor 03 and strip conductor a3; and tubular conductorc1, strip conductors a2 and 123. By this arrangement, the threeconvolutions, constituting the separate coils, are superposed one on theother, the parts being in the relation hereinbefore described. It willbe understood, of course, that, while threeseparate convolutions,connected in parallel, are shown and described, any desired number ofconvolutions may be employed, depending upon the conditions ofoperation.

Experience has demonstratedthat the cooling of the outside of the coilsby means of the tubular water cooled sections is of particularimportance in the operation of high frequency induction furnaces.Furnaces which are fed-with low frequency current may, if desired, bearranged in such a manner that the cooling tube or tubes, constitutingelements of the coil, may be disposed between the individual stripconductors forming the coil or the convolutions of the latter, but ithas been found, as a general proposition, that better results wereobtained by locating the tubular conductor members with the watercirculation on the outside of the coils, as far as possible, so that theheat generated by the other coil elements will be rapidly transmitted tothe circulating water and thus dissipated.

What I claim is:

1. An induction furnace coil having multiple convolutions, eachincluding a tubular water cooled section and a metallic band in series,the tubular sections being disposed on the outside of the coil andarranged to substantially surround the furnace.

2. An induction furnace coil, including metal band conductors andtubular water cooled conductors in series, the latter being disposed onthe outside of the coil and overlying the band conductors.

WILHELM ESMARCH.

